Permanent magnet generators are utilized in a large variety of generating systems. A typical usage requires the presence of a permanent magnet generator for generating field current for a so-called exciter. The exciter in turn utilizes the magnetic field created by the field current from the permanent magnet generator to generate more electrical power than could be generated by the permanent magnet generator itself. Such electric power is frequently multiple phase alternating current and the same is then rectified and passed to the main field winding of a main generator which in turn provides electrical power for consumption by various electrical loads.
One typical use of such a generating system is an aircraft.
Of course, such generating systems must be extremely reliable and steps must be taken to prevent disasterous failures.
One difficulty that must be faced in generating systems is the possibility of overheating and resultant destruction due to the creation of a fault, typically a short, in the electric components of the system.
In the case of a system such as described above, faults coming into existence in the exciter or main generator are relatively easily dealt with once they are detected. In the case of the development of a fault in the main field winding, it is only necessary to interrupt the source of power thereto as by disconnecting the exciter therefrom or disabling the exciter. Similarly, in the case of the development of a fault in the exciter field winding or rotor winding, it is only necessary to disrupt the flow of current from the permanent magnet generator to the exciter field. Faults developing in the main generator rotor winding can likewise be dealt with by preventing the flow of power to the main field winding or by shedding the electrical load connected to the rotor winding.
However, the task is not so easy when a fault develops in the winding of a permanent magnet generator, particularly when employed as part of an aircraft generating system or other similar generating system where the generator cannot be mechanically decoupled from the prime mover and where the prime mover cannot be shut down.
In particular, an inherent characteristic of a permanent magnet generator is the presence of stored magnetic energy which cannot be shut off by external means. Thus, where the prime mover cannot be shut down, as in the case of an aircraft engine, when a fault begins to develop, the stored magnetic energy of the permanent magnet generator will continue to induce voltage in the permanent magnet generator winding. If such winding is in the process of developing a fault, as, for example, a short, this continued generation of electrical voltage will continue to aggrevate the developing fault until the same burns free due to current resulting from the induced voltage.
The consequences of such an occurrence are dependent upon the severity of the fault but in any event are to be avoided since, at the very least, the winding carrying part of the permanent magnet generator may be destroyed beyond the point where it may be reused or rebuilt.
The present invention is directed to overcoming one or more of the above problems.